Opacity and forward scattering monitor using beam-steered solid-state light source

ABSTRACT

An improved transmissometer/particulate monitor of the type which has an optical assembly containing a solid state light source preferably a solid-state laser. The light source emits a collimated beam that is split, part of which is focused onto a reference detector that monitors the intensity of the light source, while the other part is directed to a beam-steering apparatus that causes the beam to accurately pass through a gaseous sample to a desired location such as a retro-reflector. A position-sensing detector is used in a closed-loop manner to control the beam-steering apparatus. The ratio of the total energy of the detected light beam, relative to the reference detector output, is used to determine the opacity of the gaseous sample or to provide a basis for correlation to particulate loading of the sample or both. The correlation to particulate loading is enhanced by a feature of the invention which measures the angular distribution of forward-scattered light to provide information as to the particle size distribution of the particulates.

FIELD OF THE INVENTION

This invention generally relates to transmissometers of the type whereinthe particulate concentration, optical transmission or opacity of agaseous sample is measured as a function of the attenuation of a lightbeam passed through the sample and to light-scattering nephelometerinstruments used to measure particulate concentration and size. Suchdevices are used for measuring particulate concentration, opticaltransmission, optical density and/or opacity of stacks or ducts whichcontain the gases resulting from burning fossil fuel, or process gaseswhich contain particulates from industrial processes.

DESCRIPTION OF THE PRIOR ART

The federal government of the United States has set limits as to theamount of particulates and other pollutants that an electric utility orother business may emit into the air. In addition, there are limitationson visible emissions, also known as opacity. Typically these emissionsare determined from measurements of the stack gasses as they flowthrough the stack. Opacity monitors, which are also calledtransmissometers, have been used as correlation devices to monitor themass concentration of dust and other particulates passing through astack or other conduit. Opacity is determined by passing a light beamthrough the stack gasses and determining the difference between theintensity of the original light beam and the intensity of the lightwhich strikes a detector after having passed across the stack. In singlepass monitors the detector is placed on the stack wall opposite thelight source. In double pass systems a retro-reflector is located on thestack wall opposite the light source and reflects the light back throughthe stack to a detector positioned near the light source.

Most of the transmissometers of the prior art for stacks and ducts usean incandescent lamp as the optical source. This posed a number ofshortcomings which are described in the U.S. Pat. No. 4,937,461, whichdiscloses the use of a solid-state light source that emits light beamshaving a wavelength between 500 and 600 nanometers as the opticalsource. In recent years, the companies Land Combustion, Sick OpticElectronic, Codel, and Phoenix Instruments have all sold opacitymonitors that use solid-state lamp technology. All of these prior artopacity monitors, whether using incandescent or solid-state lamps,require large mounting ports on the stack. The reason for this isrelated to the need to maintain alignment of the projected beam onto theretro-reflector on the other side of the stack or duct. Such systems areprone to alignment shifts due to thermal expansion and contraction ofthe stack and the hardware holding the light source, detector andretro-reflector. The mounting ports which hold these monitors on a stackmust be large enough to allow for alignment tolerances. However, themounting ports must be continuously purged with clean air, for the dualpurposes of protecting the apparatus and avoiding blockage withparticulate deposits. The large amount of purge air required by theselarge mounting pipes requires the use of large and expensive air blowerswhich in turn must be fitted with air filters to remove dust from theambient air. Periodic replacement of these filters, cleaning of opticalsurfaces that have been fouled by imperfectly filtered purge air,condensation due to humidity in the purge air, and equipment damage frompurge blower failure, constitute most of the maintenance associated withtransmissometers. The preferable solution to all these problems would beto use dry, filtered instrument air, such as is commonly available atsites where transmissometers are used. However, the large amount,typically 20-60 cubic feet per minute, of purge air that is required toclean the large mounting ports prohibits this option.

Another family of transrissometers which use either solid-state lasersor helium-neon gas lasers has been sold by MIP and KVB. Theseinstruments are single-pass transmissometers wherein the laser source isprojected into one side of a stack or duct and a detector is located atthe opposite side. One of the useful features of this device is that,because the laser beam is both intense and narrow, the instrument can beaffixed to a stack or duct using small mounting ports, which in turnrequires the use of less purge air to keep the ports free of particulatematerial from the gas stream being monitored. This permits the use ofinstrument air supplies which are commonly available at the job-site,rather than expensive blowers and the attendant maintenance of airfilters. An additional advantage is that because laser beams are highlydirectional, a laser-based system can, in principle, be used over longmeasurement paths that would be difficult using less intense solid-statesources such as LEDs that diverge. However, this approach of using anarrow laser beam poses a severe difficulty inasmuch as the beam must bekept aligned onto the detector. For instance, if the laser beam is 0.25"in diameter and must be kept onto the active area of a detector that is2" in diameter and 40 feet away from the laser source, the requiredalignment tolerance is ±0.15 degrees. This is not achievable in acost-effective manner in many applications. A second disadvantage withthis prior design is that it is a single-pass approach, requiringelectronics on both sides of the stack or duct being monitored. Finally,the helium-neon laser generates large amounts of heat, is not rugged, isshort-lived, and is expensive to replace.

A disadvantage common to all of the prior art involves the issue ofcalibration checks. The United States Environmental Protection Agency(EPA) requires that an opacity monitor that is being used to demonstratecompliance with visible emission standards be equipped with a mechanismthat can be used to simulate a condition of zero particulate or zeroopacity, and a condition equivalent to a predetermined upscale opacityor particulate concentration. These must be performed, at a minimum,once every 24 hours. Even when the transmissometer is not being used forEPA compliance, the user prefers to have this feature as a part of thesystem. The prior art typically achieves the zero condition byinterposing into the projected light beam a reflective surface whichsimulates the effect of the retro-reflector under clear stackconditions. An upscale condition is simulated by interposing an opticalfilter between the sensing optics and the zero simulation mirror. In theprior art, implementation of this calibration check typically requiresthe use of additional mechanical moving parts such as solenoids, motors,bearings, and electrical relays.

A related disadvantage of the prior method of performing a zero andupscale calibration check is that, when the measurement pathlength of atransmissometer is changed, the gain factors of the instrument need tobe adjusted. This, in turn, requires changing the zero surface such thatthe optical energy reflected by the zero device is now equivalent to thecross-stack distance at the new calibration distance. This is most oftenaccomplished by manual adjustment of an iris that alters the crosssection of the zero reflective surface.

Another disadvantage of all the prior art is that, since maintenance ofalignment of the projected beam onto the retro-reflector is necessary toan accurate measurement, rigid and accurate mounting pipes are required.However, in many applications, the desired installation location is onthe sides of metal stacks and ductwork which are not mechanically rigid.A significant part of the expense of installing this kind of equipmentis associated with adding stiffeners, braces, and the like to existingstacks and ductwork in order to maintain alignment.

In addition to the zero and upscale check, the EPA standard for visibleemissions monitoring requires a linearity check whereby two additionaloptical filters, corresponding to two additional upscale opacity values,are inserted into the measurement path. This is required to be done on aperiodic basis, such as on the calendar quarter, and/or following repairor recalibration of the instrument. In most of the prior art systems,this must be done manually.

The most common use of transmissometer technology outside of the UnitedStates is for monitoring particulate concentration (e.g., milligrams ofparticulate per cubic meter of gas). It is obvious that when theparticulate concentration increases in a conduit, with no changes inparticulate composition, density, or size distribution, the attenuationof the beam of light passing through the conduit, and hence the opacity,will increase. However, in many cases, the causal factors that changethe particulate concentration also change other particulate properties,most notably the size distribution. In typical smokestacks, for a fixedmilligram per cubic meter concentration, the optical attenuation isinversely proportional to the average particle size. It follows that, ifa transmissometer is being used for correlation to particulateconcentration, even an approximate measurement of particle size canprovide significant improvement in the accuracy of the calculation ofconcentration.

Consequently, there exists a need for a transmissometer which uses anarrow-beam solid-state light source with an automated means to sensethe projected angle of the beam and maintain the alignment of the beamonto a retro-reflector. The system should be able to perform a zerocalibration check and up to three upscale calibration points without theneed for additional moving parts. In addition, for those cases in whicha transmissometer is being used for correlation to particulateconcentration, a means of determining particle size distribution isneeded to correct the opacity-to-concentration correlation function.

SUMMARY OF THE INVENTION

We provide an opacity and forward scattering monitor containing asteerable, solid state light source which requires low power, is lightand rugged, gives off low heat, has long life and can be directlymodulated and run on reduced-duty cycling. This light source iscollimated into a narrow axial beam and is dynamically steered so as tomaintain alignment while overfilling a small retro-reflector. Thisconfiguration permits use of mounting pipes of typically 1.5" (3.8 cm.),as compared to 3.5" (8.9 cm.) to 6" (15.2 cm.) pipes required by theprior art. Since the volumetric flow required to purge a pipe goes asthe square of the diameter, our monitor requires only 5 to 10 cubic feetper minute (2 to 5 liters per second) of purge air, which can be readilysupplied with instrument air. Maintenance of filters and blowers isthereby completely eliminated. Because our invention is self-aligning,it can be installed on stack surfaces and ductwork that are notespecially rigid, without the need for support structure modification orfrequent realignment by a technician.

One embodiment of our monitor steers the optical beam using rotatingcylindrical prisms. Normally, the control of beam position using thesedevices is complex because the effect of moving one prism is dependenton the current position of the other prism. However, by tilting theoptical assembly, it is possible to create an optical situation in whichindependent movement of either prism results in movement of the beam inmutually orthogonal directions. This permits use of standard quaddetectors and a simple control algorithm.

We further prefer to provide that, in addition to the ability to keepthe beam of light centered on a retro-reflector, the mechanism is ableto direct the beam in any other desired direction relative to the centerof the retro-reflector. By moving the beam to various known angles awayfrom the retro-reflector and then measuring the amount of scatteredenergy from the beam that nonetheless reaches the retro-reflector, wecan create a distribution profile of the light scattered byparticulates. The distribution of scattered light is related to the sizedistribution of the particles, which we then use to calculate thecorrelation function used to calculate particulate density from themeasured opacity.

The ability of the instrument to steer the optical beam also simplifiesthe zero and upscale calibration checks. Rather than interposing a zeroreflector and optical filters into the beam path, our system directs theoptical beam onto pre-calibrated surfaces of known reflectivity. Thus,no additional moving parts are required other than those already used tosteer the beam. Furthermore, the calibration target is shaped such that,by moving the steered beam slightly, the cross section of the targetthat is within the beam can be adjusted, thereby eliminating the needfor the adjustment iris that is required by the prior art.

Our monitor is useful for measuring particulate concentration, opticaltransmission or opacity of stacks or ducts for which prior opticallight-scattering instruments, such as are used for determination of inbenign environments, have not heretofore been successfully applied. Ourdevice can be used to measure corrosive, hot, vibration-proneenvironments within large utility and industrial stacks and ductwork.

Other objects and advantages of our monitor will become apparent from adescription of the certain present preferred embodiments show in thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view partially in section of a present preferredembodiment of our monitor mounted on a portion of a stack or duct.

FIG. 2 is a side view partially in section of the optical assembly ofthe embodiment of FIG. 1.

FIG. 3 is a sectional view taken along the line III--III of FIG. 2.

FIG. 4 is a side view partially in section of the beam steeringmechanism of the embodiment of FIG. 1.

FIG. 5 is an end view of the beam steering mechanism of FIG. 4 takenalong the line V--V.

FIG. 6 is a sectional view showing the quad detector that was takenalong the line VI--VI in FIG. 2.

FIG. 7 is a side view partially in section of the retro-reflectorassembly of the embodiment of FIG. 1.

FIG. 8 is an optical diagram showing the light path which the beamsteering mechanism is in a zero position.

FIG. 9 is an optical diagram similar to FIG. 8 showing the light pathwhich the beam steering mechanism is in a full up position.

FIG. 10 is an optical diagram similar to FIG. 8 showing the light pathwhich the beam steering mechanism is in a full down position.

FIG. 11 is a side view partially in section of a second presentpreferred embodiment of our monitor mounted on a portion of a stack orduct.

FIG. 12 is a side view partially in section of the retro-reflectorassembly of the embodiment of FIG. 11.

FIG. 13 is a sectional view taken along the line XIII--XIII in FIG. 12.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, our system has a retro-reflector assembly 1 and anoptical assembly or main assembly 7 which are mounted on opposite sidesof a conduit 2. This conduit may be a stack or duct containing the gasesresulting from burning fossil fuel, or process gases which containparticulates from industrial processes such as, but not limited to wasteincineration, lime kilns, petrochemical processes, food processing, andmeasurement of dust in mine shafts or vehicular tunnels. The conduit 2illustrated in the drawings is shown to be quite small. This was doneonly for illustrative purposes as we expect our monitor to be used inconduits ranging in size from a less than a meter to over 40 meters indiameter.

A collimated beam of light is projected from a light source 30 inassembly 7, through the conduit 2 in which the opacity or particulate isto be measured, and to a retro-reflector assembly 1 which returns thebeam of light to the optical assembly 7. Retro-reflector assembly 1 alsoserves the function of providing the optical information used by thebeam-steering mechanism 10 within assembly 7. We prefer to use asolid-state laser as the light source. For applications involvingmeasurement of opacity to comply with United States EnvironmentalProtection Agency Regulations, the light source is required to have apeak and mean spectral output between 500 and 600 nanometers. A suitablelaser for this application is made by Brimrose, and sold under ModelNumber BWT-1-E.

FIG. 2 shows the main assembly 7 in more detail. A light beam indicatedin chain line from solid state laser 30 is reflected off beam-splitter35 to second beam-splitter 34, through beam expander 11 and tobeam-steering apparatus 10. From the beam-steering apparatus 10, thebeam normally traverses the stack 2. We prefer to provide a 0.75" (1.9cm.) diameter beam coaxially aligned with retro-reflector assembly 1,overfilling a retro-reflector 20, which is 0.25" (0.64 cm.) diametershown in FIG. 7. Retro-reflector 20 returns the beam through thesteering apparatus 10, the beam expander 11 and beam-splitter 34 tobeam-splitter 36 which reflects 80% of the beam energy to signaldetector 32. Detector 32 preferably is a quad detector able to sense thedistribution of the beam on one of four symmetrical detector surfaces.FIG. 6 shows such a situation, in which the projected beam indicated bybroken circle 40 is focused onto detector 32 as off-axis, being in thenegative x, positive y direction. Separate measurement of positionsignals from the four elements of quad detector 32 provides feedbacksignals for control of beam-steering apparatus to bring the focused beamto the desired position 41. Addition of the four signals from the fourquadrants of the detector 32 gives information about total attenuationof the beam across the stack, which is a measure of opacity.

Beam-splitter 36 preferably is selected to allow 20% of the beam energyto be focused onto an eyepiece 31, allowing observation of thealignment. The portion of the beam that is transmitted throughbeam-splitter 35 is focused into reference detector 33 and is used toestablish the ratio of the transmitted beam intensity as determined bythe total signal from quad detector 32 to the laser intensity asdetermined by reference detector 33.

As illustrated in FIGS. 4 and 5, beam-steering apparatus 10 consists oftwo similar assemblies, each containing one of two wedge prisms 22a and22b. Each prism is axially aligned with beam expander 11. As can be seenis FIG. 5, there is a servo motor 24, timing belt 29 and encoder 25associated with each prism. Each prism can be independently rotated viabearings 23, timing belt 29 driven by servo motor 24 and encoder 25 Eachprism is associated with a homing switch 26 which is used as a referencepoint for the encoder 25. Homing switch 26 is activated once perrevolution of the associated prism 22a or 22b. Each encoder is relatedto a prism via a 4:1 gear ratio such that the encoder rotates 4 timesper prism rotation. Each encoder 25 has three square-wave outputs: (1) aonce-per-encoder pulse, (2) a 0°-phase pulse for each 0.18 degrees ofrotation of the encoder, and (3) a 90°-phase pulse, that lags the0°-phase pulse by a quarter-cycle, for each 0.18 degrees of rotation ofthe encoder. The 0° and 90° phase pulses indicate which direction theencoder is turning. The encoder resolution, combined with the 4:1 gearratio, enables the system to know the rotation of the prisms by ±0.045°.

We prefer to use wedge prisms which are able to cause a deflection of 6°in the direction of the beam passing through it. Rotating either prismabout 360° causes the expanded laser beam to describe a circlecorresponding to a 6° deflection of the beam. By independentlycontrolling the motion of the two prisms, the beam can be steered to anydirection inside a 12° deviation from its original direction. Since thesystem can identify prism rotation to within ±0.045°, and since a 180°prism rotation creates a change of direction of 6° in the projectedbeam, it follows that we are able to identify the beam direction towithin ±(0.045/180)(6°)=±0.0015°. At a distance of 130 feet, or about 40meters, this corresponds to a beam deflection of (130 feet)tan(±0.0015°)=±0.034 inches or ±0.086 cm. This is more than adequateresolution for maintaining the beam onto the retro-reflector.

FIG. 8 shows the situation in which the prisms are anti-aligned so as toproduce no change in the direction of the beam. This is referred to asthe Home Position of the beam. FIG. 9 shows the situation in which thetwo prisms are aligned with the thick edges of the prisms verticallyaligned upward. We call this the Full Up direction of the beam whichstrikes surface 39 on the retro-reflector assembly or stack wall. Whenwedge prisms are used that create a deflection of 6° angle Θ will be 12°above the optical axis. FIG. 10 shows the situation in which the twoprisms are aligned with the thick edges of the prisms vertically aligneddownward. We call this is the Full Down direction of the beam. Whenwedge prisms are used that create a deflection of 6° angle Θ will be 12°down from the optical axis.

If, from the Full Up position of FIG. 9, the two prisms are rotated inopposite directions, one clockwise and one counter-clockwise, by 45°, itis straightforward trigonometry to show that the beam will be steered toa direction 8.5° vertically above the Home Position. It is also apparentthat, because the thick side of each prism is now at a 45° angle withrespect to the vertical and horizontal planes, that a small rotations ofeither prism, with the other held stationary, will produce a movement inthe projected beam that is at approximately a 45° angle with respect tothe vertical and horizontal planes. The effect of individual movingPrism 22a will, in addition, be orthogonal to the effect of moving prism22b. Referring again to FIG. 6, the quad detector 32 can be orientedwithin the optical system such that the detector's x-axis ispredominately associated with movements of prism 22a while thedetector's y-axis is predominately associated with movements of prism22b. This makes possible the development of simpler control algorithmsin the region of 8.5° beam direction than is possible in the near-regionof other beam directions.

As shown in FIGS. 1 and 2, we prefer that the entire apparatus of laser,beam expander, steering apparatus, detectors and beam-splitters betilted downward at an angle Δ of 8.5° with respect to the axis formed bythe center of the front window 38 and retro-reflector 20 in theretro-reflector assembly 1. This results in a configuration such thatwhen the beam is pointing out of the ±1° cone angle defined by themounting nozzle 3, prisms 22a and 22b will be oriented such that themutually orthogonal effect of individual rotations will apply.

As seen most clearly in FIG. 2, the laser, beam-splitters, beam expanderand detectors are affixed to an optical bench 12 that preferably is aprecision casting. The components, along with the beam-steeringapparatus 10, are affixed to a base-plate 9 mounted on a plate 8 whichalso contains the front window 38. Intermediate housing 51 providesaccess to a calibration target to be described later. Purge housing 52includes provision for a purge port 53 connected to nozzle 3 whichprovide protection of the window 38 and calibration target 13 from heatand gases. Purge air is supplied through line 54 that preferably isconnected to a source of dry, filtered, instrument air. The overallapparatus is designed so that the optical assembly 7 and theretro-reflector assembly 1 each fit onto a system flange 4 whichcontacts a seal 5. The beam steering assembly is preferably controlledby a microprocessor (not shown) positioned within the optical assembly.Power and communication lines (not shown) for the microprocessor,motors, encoders light source and detectors are provided throughconduits 55. Purge air for the retro-reflector is provided throughconduit 56. The entire assembly is protected from rain, dust and otherfactors by a cover 14, having a window 37 opposite eyepiece 31.

FIG. 3 is an inboard view of the intermediate housing 51. Thecalibration target as depicted shows in chain line alternate positionsof the beam 107 as a 1.5" projected spot which may be steered to one offive locations on the target. One of the five locations is a flat blackor other absorbing surface 104 which serves to simulate the situation of100% opacity in the conduit, or infinitely high particulate loading inthe conduit which does not permit any of the beam to cross the conduit.Each of the other four calibration targets includes a reflective disc106 and reflective rectangle 105 which, when co-illumninated by thebeam, reflect back an amount of energy approximately equal to the energythat would be reflected from retro-reflector 20 under clear-conduitconditions. By adjusting the steering of the beam to include more orless of reflective rectangle 105, the equivalency of the calibrationtarget and cross-conduit retro-reflector 20 can be achieved with anydesired degree of accuracy. This feature of our invention eliminates theneed for mechanical adjustment of a calibration iris. A linearity check,to any desired upscale opacity, can be achieved by affixing neutraldensity filters of known optical transmission, to calibration targets.

As shown in FIG. 3, the centers of the four reflective discs 106, aswell as the centerlines of the four reflective rectangles 105 arearranged symmetrically about the Home Position of the beam. This meansthat, once the prism positioning needed to establish equivalency withretro-reflector 20 is established the other four calibration locationscan be located by the simple procedure of rotating both prisms by anumber of degrees equivalent to the angular separation of the targetsfrom the home position.

Additional details of the retro-reflector assembly 1 are shown as FIG.7. Flange 4, seal 5, and purge nozzle 3 are equivalent to those in FIG.2. A purge air sensing switch 27 is shown monitoring the pressure dropacross the purge venturi.

Evaluation of the opacity caused by particulate in the stack providesonly partial information about particulate concentrations. Additionalinformation is achieved by measuring and evaluating the angulardistribution of light scattered from the beam. This can be done by thesecond preferred embodiment of our monitor shown in FIGS. 11, 12 and 13.FIG. 11 depicts the situation in which the system first aligned the beamacross the stack by using signals from the quad detector 32 to enablethe beam-steering apparatus 10 to center the beam on and aboutretro-reflector 20. Both opacity and near-forward scattered light can bemeasured in this configuration. The beam steering apparatus 10 now usesinformation from the encoder 25 to point the beam at one of a number ofdesired angles such that no energy from the primary 1.5" beam isentering nozzle 3. The only optical energy entering nozzle 3 will be dueto scattered light from particulates in the conduit 2 as indicated byparticle P in FIG. 11. Note that the size of purge nozzle 3 on thesource side of the conduit is made larger to enable larger steeredangles to be achieved. Preferably angles of from 0° to 6° will be used.

The details of how the scattered light is measured can be understoodfrom FIGS. 12 and 13. Retro-reflector 20 is carried in a housing 42 thatis bonded to the center of a glass plate 43. When the diameter of thelight beam from the solid state light source is larger than theretro-reflector or that beam is directed to a position adjacent housing42 light will enter into a Cassegrain Telescope 110. That light willstrike primary mirror 46 and be reflected to secondary mirror 47 locatedbehind the retro-reflector 20. This folded annular beam of scatteredenergy is thereby focused onto one end of fiber optical cable 111. Theother end of cable 111 is connected to an analyzer (not shown) where theinformation is processed, preferably by the same microprocessorcircuitry as is used for the opacity measurement. This information,taken over scattering angles of from 0° to 6°, permits more accuracy ofthe correlation of opacity to actual mass loading of particulates. Ifdesired, quad detector 32 could be used to detect backscattered lightfrom the same particle P. This will, in some cases, necessitateinstallation of a light trap at the location where the beam strikes thefar wall of the conduit.

We prefer to provide electronic and microprocessor components andsoftware programs in the electronic module 15, shown in FIG. 2, toprovide automatic alignment, to enable the system to re-establishalignment following power failures, and to provide means for automaticchecks of system calibration without the need of moving parts other thanthe beam-steering mechanism. Electronic module 15 includes a drivecontroller 16 which can be used to operate the laser 30 in a pulsed modeso as to reduce the fraction of time that the laser 30 is operating soas to extend laser life. In typical operation for opacity monitoring orlight-scattering measurements, the laser is operated in a 50% dutymodulation mode for 200 milliseconds out of every second, for aneffective duty of 10%, while providing a useful signal as a light beamfor applications only requiring response times of a few seconds.

Although we have shown and described certain present preferredembodiments of our monitor it should be distinctly understood that ourinvention is not limited thereto but may be variously embodied withinthe scope of the following claims.

We claim:
 1. A monitor comprised of:a. a housing sized and configuredfor attachment to a conduit and having a window through which light maypass; b. a solid state light source within the housing; c. a beamsteering unit within the housing and having a pair of colinear rotatablewedge prisms positioned to direct a light beam from the light sourcethrough the window and across the conduit; and d. at least one detectorpositioned to receive the light beam from the solid state light sourceafter that light beam has passed through the conduit and to receivelight which has been forward scattered after striking particles in theconduit which detector produces a signal corresponding to intensity oflight received by the detector the signal being useful for measuring atleast one of opacity of a medium passing through the conduit and lightscattered by particulates in the conduit.
 2. The monitor of claim 1 alsocomprising a reference detector positioned within the housing to receivea reference light beam from the solid state light source.
 3. The monitorof claim 2 also comprising a beam splitter positioned within the housingfor splitting a beam from the solid state light source into a referencebeam and a measuring beam and directing the measuring beam through theconduit and the reference beam to the reference detector.
 4. The monitorof claim 2 wherein the at least one detector is a quad position detectorand electronically connected to the reference detector and the beamsteering unit.
 5. The monitor of claim 1 wherein the detector is withinthe housing and also comprising a retro-reflector positioned to reflectback through the conduit to the detector a light beam from the solidstate light source which has passed through the conduit.
 6. The monitorof claim 5 also comprising a second signal detector and a beam splitterpositioned to split the beam which has passed through the conduit into afirst beam directed to the second signal detector and second beamdirected to the at least one detector.
 7. The monitor of claim 5 alsocomprising detection optics positioned near the retro-reflector whichdetection optics can detect scattered light from the light beam when thelight beam is aimed at a known angle relative to the detection optics.8. The monitor of claim 7 wherein the detection optics comprise aCassegrain telescope.
 9. The monitor of claim 8 also comprising a fiberoptic cable connected between the Cassegrain telescope and the housing.10. The monitor of claim 5 wherein the beam steering unit is comprisedof a pair of rotatable wedge prisms in an optical path of the light beamsuch that the light beam is directed to the retro-reflector when theprisms are orthogonal to one another, as a result of which independentmotion of either prism will cause orthogonal movement of the light beamfrom the solid state light source.
 11. The monitor of claim 1 whereinthe beam steering unit also comprises a first servo motor connected torotate one wedge prism and a second servo motor connected to rotate theother wedge prism.
 12. The monitor of claim 11 wherein the beam steeringunit also comprises a first encoder connected to the first wedge prismand a second encoder connected to the second wedge prism.
 13. Themonitor of claim 1 also comprising a calibration assembly having atleast one region of a known reflectivity which assembly is positionedwithin the housing so that the beam steering unit directs a light beamonto the at least one region for reflection to the detector.
 14. Themonitor of claim 1 wherein the solid state light source is a laser. 15.The monitor of claim 14 also comprising a drive controller attached tothe laser for turning the laser on and off in a sequence which willprovide a useful light beam for at least one of opacity monitoring andforward scatter monitoring thereby providing reduced duty operation forextended laser life.
 16. The monitor of claim 1 wherein the solid statelight source has peak and mean spectral response between 500 and 600nanometers.
 17. A monitor comprised of:a. housing sized and configuredfor attachment to a conduit and having a window through which light maypass; b. a solid state light source within the housing; c. a beamsteering means within the housing and positioned for directing a lightbeam from the light source through the window and across the conduit;and d. at least one detecting means positioned for receiving the lightbeam from the solid state light source after that light beam has passedthrough the conduit and for receiving light which has been forwardscattered after striking particles in the conduit which detectorproduces a signal corresponding to intensity of light received by thedetector the signal being useful for measuring at least one of opacityof a medium passing through the conduit and light scattered byparticulates in the conduit.
 18. The monitor of claim 17 wherein thedetecting means is within the housing and also comprisingretro-reflecting means for reflecting back through the conduit to thedetecting means a light beam from the solid state light source which haspassed through the conduit.
 19. The monitor of claim 18 also comprisingdetection optic means positioned near the retro-reflecting means whichdetection optic means can detect scattered light from the light beamwhen the light beam is aimed at a known angle relative to the detectionoptic means.